20655-48-5Relevant articles and documents
Effect of changing electrophilic center C=O to C=S on rates and mechanism: Pyridinolyses of O-2,4-dinitrophenyl thionobenzoate and its oxygen analogue
Um, Ik-Hwan,Han, Hyun-Joo,Baek, Mi-Hwa,Bae, Sun-Young
, p. 6365 - 6370 (2007/10/03)
Second-order rate constants have been measured spectrophotometrically for the reactions of O-2,4-dinitrophenyl thionobenzoate (1) and 2,4-dinitrophenyl benzoate (2) with a series of substituted pyridines in 80 mol % H 2O/20 mol % DMSO at 25.0 ± 0.1 °C. The Bronsted-type plots obtained are nonlinear with β1 = 0.26, β2 = 1.07, and pKa° = 7.5 for the reactions of 1 and β2 = 0.40, β2 = 0.90, and pKa° = 9.5 for the reactions of 2, suggesting that the pyridinolyses of 1 and 2 proceed through a zwiterionic tetrahedral intermediate T± with a change in the rate-determining step at pKa° = 7.5 and 9.5, respectively. The thiono ester 1 is more reactive than its oxygen analogue 2 except for the reaction with the strongest basic pyridine studied (pK a = 11.30). The k1 value is larger for the reactions of 1 than for those of 2 in the low pKa region, but the difference in the k1 value becomes negligible with increasing the basicity of pyridines. On the other hand, 1 exhibits slightly larger k2/k -1 ratio than 2 in the low pKa region but the difference in the K2/k-1 ratio becomes more significant with increasing the basicity of pyridines. Pyridines are more reactive than alicyclic secondary amines of similar basicity toward 2 in the pKa above ca. 7.2 but less reactive in the pKa below ca. 7.2. The k1 value is slightly larger, but the k2/k-1 ratio is much smaller for the reactions of 2 with pyridines than with isobasic secondary amines in the low pKa region, which is responsible for the fact that the weakly basic pyridines are less reactive than isobasic secondary amines.
Nonlinear Broensted-Type Plot in the Pyridinolysis of 2,4-Dinitrophenyl Benzoate in Aqueous Ethanol
Castro, Enrique A.,Santander, Carlos R.
, p. 3595 - 3600 (2007/10/02)
From the kinetics of the title reaction in 44 wtpercent aqueous ethanol at 25 deg C, ionic strength 0,2 M (KCl), a curved Broensted-type plot is obtained.The curve is interpreted in terms of a zwitterionic tetrahedral intermediate in the reaction path and a change in the rate-determining step from breakdown to products of this intermediate to its formation as the pyridine becomes more basic.A semiempirical equation based on this hypothesis accounts for the experimental results.The center of the Broensted-type curvature (pKa0) is located at pKa = 9.5, lower than the value found in the pyridinolysis of 2,4-dinitrophenyl p-nitrobenzoate in the same solvent, showing that electron withdrawal from the acyl group of the substrate favors amine expulsion relative to phenoxide from the tetrahedral intermediate.It is claimed that the influence of the acyl group of the substrate on the value of pKa0 cannot be quantified when comparing similar reactions in water and aqueous ethanol since the pKa0 value for a given reaction should be larger in the latter solvent.The activation parameters obtained support the hypothesis that for the title reactions, pKa0 > 9.
